If you’re a budding software developer trying to decide which programming skills would be most valuable to devote your time to learning - or, if you’re a seasoned pro just looking become more marketable - Quartz had some interesting news for you last week. Using U.S. job listing data collected by Burning Glass and the Brookings Institution, Quartz found that the most valuable programming skill to have today is Ruby on Rails (AKA Rails or RoR) experience, with an average salary of $109,460. However, before you run out and buy Ruby on Rails for Dummies, you might want to consider some other data which indicate that Rails (and Ruby) usage is not trending upwards.

The Quartz findings were based on 3.3 million U.S. job listings, across 52,000 companies from the first quarter of 2013 collected by Burning Glass and Brookings. Brookings produced a larger report using them looking at the general demand for STEM workers, while Quartz homed in on the demand for workers with programming-specific skills. The Quartz results are based on a large data set, to be sure, but they’re also for a point in time that was almost two years ago. I wondered, then, if there were more recent data to show that demand for Rails engineers is still as strong and whether the trend is going up, down or remaining flat?

First, I turned to MS Gooroo, which has collected data from over 300,000 job listings in the U.S., U.K. and Australia. Not as big of a data set as the Burning Glass data, but still good sized. The average salary for U.S. jobs mentioning Rails in July, 2014 was $87,725, more than that for language heavyweights like Java ($84,259), JavaScript ($80,039) and C ($81,166). This confirms the Quartz findings that Rails skills are particularly well compensated.

But, I’m interested in trends. While MS Gooroo only presents salary data for a point in time (July, 2014), they do have trends over time for the demand for certain skills. They found that the percentage of U.S. job listings mentioning Rails in July 2014 was 1.1%, which was down from 1.8% in December 2013, an almost 40 percent drop. While the pay for Rails engineers is high, demand over the last year seems to be dropping.

I wanted to get a better sense for how much Rails is being used by developers and how that’s changed since the beginning of 2013. However, since Rails is a framework, and not a programming language, it’s harder to come by the same kind of usage data that’s available for languages. I decided, then, to look at the trend in the usage of Ruby, the language upon which Rails was built, as a proxy.

GitHut, which ranks languages quarterly by the number of active repositories, has Ruby ranked 5th in Q3 2014, down from 2nd in Q1 2013. In terms of the percentage of active GitHub repositories, Ruby has been dropping steadily for the last two years, down from 10% in Q1 2013 to 6.7% in the most recent quarter.

What should developers make of all this then? Three things, in my opinion:

As Quartz found, Ruby on Rails is a valuable skill which can lead to a high paying job.

Demand by U.S. employers for engineers with Rails skills, however, has been on the decline, at least for the last year.

If use of the Ruby programming language itself can be considered a reliable proxy for the use of Rails, its use by engineers has also been dropping at least moderately (and, looking at GitHub data, significantly) since the beginning of 2013.

Ruby on Rails, then, is certainly still a valuable skill to have these days. But, it’s not clear right now if that will that continue to be the case in the years to come. Just to be safe, go ahead and pick up a copy of Java for Dummies while you’re at the bookstore (er, on Amazon).